Method for steering a smart antenna for a WLAN using a self-monitored re-scan

ABSTRACT

A smart antenna steering algorithm performs a self-monitored re-scan during a sustained use period after having selected a preferred antenna beam. During a sustained use period, a re-scan of the other antenna beams is not performed. The steering algorithm periodically monitors a quality metric of the ongoing radio link provided by the preferred antenna beam. The quality metric is based upon a signal quality metric and a link quality metric. If the quality metric drops below certain thresholds during the sustained use period, the steering algorithm either swaps the preferred antenna beam with an alternate antenna beam or initiates a re-scan of the available antenna beams for selecting a new preferred antenna beam.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/553,902 filed Mar. 17, 2004, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of wireless local areanetworks (WLANs), and more particularly, to an antenna steeringalgorithm for a smart antenna operating in a WLAN.

BACKGROUND OF THE INVENTION

Smart antenna technology is directed to antennas having the ability tochange radio beam transmission and reception patterns to suit theenvironment within which radio communication systems operate. Smartantennas have the advantage of providing relatively high radio link gainwithout adding excessive cost or system complexity.

Smart antenna technology has been used in wireless communication systemsfor decades, and has recently been investigated for use in wirelesslocal area networks (WLANs). In a WLAN, a client station (CS) is adevice used by a mobile end user for communication with other stationswithin the same WLAN or with other entities outside of the WLAN. Centralhubs that provide distribution services in WLANs are referred to asaccess points (APs). Access points are similar to base stations inwireless telecommunication systems.

A client station can be equipped with a smart antenna as well as anantenna steering algorithm that enables the antenna to switchelectronically to a particular directional antenna beam. This enablesthe client station to communicate with its access point while achievinghigh performance.

Signal quality information, such as a received signal strength indicator(RSSI) or a signal-to-noise ratio (SNR), is typically used to determineor steer a preferred directional antenna beam. However, it is difficultto accurately measure signal quality information when the receivedsignal includes undistorted signals plus random noise. In addition, thereceived signal itself may be distorted and directional interference maybe added in the received signal. Consequently, signal qualityinformation alone may not always be a reliable indicator of the qualityof the radio link. This is especially true in radio environments thatare rich with interference coming from other client stations and accesspoints, or other types of noise and interference sources.

Once a directional antenna beam has been selected for a smart antenna,the quality of the radio link may change if the client station moves oran object moves into the radio link path. As a result, the selecteddirectional antenna beam may no longer be the preferred antenna beam. Tomaintain communications, the other available antenna beams may bere-scanned for selecting a new preferred antenna beam. As noted above,the signal quality information alone may not always be a reliableindicator of the quality of the radio link when re-scanning.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a method for steering smart antenna beamsin a wireless local area network (WLAN) while more accurately takinginto account the quality of the radio links.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a method for operating a clientstation in a WLAN communication system comprising an access point, withthe client station comprising an antenna steering algorithm and a smartantenna responsive to the antenna steering algorithm for selecting oneof a plurality of antenna beams. The method comprises selecting apreferred antenna beam and an alternate antenna beam, exchanging datawith the access point using the preferred antenna beam during asustained use period during which a re-scan of the other antenna beamsis not performed, and periodically calculating during the sustained useperiod a quality metric of the exchanged data for the preferred antennabeam.

The calculating may comprises determining at least one link qualitymetric (LQM) of the exchanged data for the preferred antenna beam,determining a signal quality metric (SQM) of the exchanged data for thepreferred antenna beam, and combining the at least one LQM and the SQMfor calculating the quality metric. The at least one LQM advantageouslyimproves the antenna steering decision in addition to the SQM,particularly when the exchanged data may be distorted by random noise.

The quality metric for the preferred antenna beam is compared to a swapthreshold range. The preferred antenna beam is swapped with thealternate antenna beam if the quality metric is within the swapthreshold range to continue exchanging data with the access point withinthe sustained use period. If the quality metric is not within the swapthreshold range, then the method further comprises comparing the qualitymetric to a re-scan threshold for initiating a re-scan of the pluralityof antenna beams for selecting a new preferred antenna beam.

The at least one LQM may be based upon at least one estimate of a frameerror rate (FER) of the exchanged data. The at least one LQM maycomprise a downlink LQM and an uplink LQM. A weighting factor may beused when combining the downlink LQM and the uplink LQM.

The WLAN may comprise an 802.11 WLAN and the client station comprises amedia access control (MAC) layer including a plurality of frame countersfor estimating frame error rates of the exchanged data. A first set ofcounters may be used for determining the downlink LQM and a second setof counters may be used for determining the uplink LQM.

As an alternative, the LQM may be based upon a transfer rate of theexchanged data for the corresponding antenna beam. The transfer may bedefined by a throughput and/or a data rate of the exchanged data for thecorresponding antenna beam.

The SQM may be based upon a received signal strength indicator (RSSI) ofthe exchanged data. A weighting factor may also be used when combiningthe LQM and the SQM. The antenna beams may comprise a plurality ofdirectional beams and an omni-directional beam.

Another aspect of the present invention is directed to a client stationfor operating in a WLAN communication system comprising an access point.The client station may comprise a switched beam antenna for generating aplurality of antenna beams, a beam switching unit coupled to theswitched beam antenna for selecting one of the plurality of antennabeams, and a transceiver coupled to the beam switching unit forexchanging data with the access point via a selected antenna beam. Anantenna steering algorithm module executes the above described antennasteering algorithm for performing a self-monitored re-scan forexchanging data between the transceiver and the access point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram of an 802.11 wireless local area network(WLAN) including an access point, and a client station operating with asmart antenna in accordance with the present invention.

FIG. 2 is a block diagram of the client station illustrated in FIG. 1.

FIG. 3 is a flowchart for computing signal quality metrics (SQM) andlink quality metrics (LQM) used in selecting antenna beams in accordancewith the present invention.

FIG. 4 is a flowchart for operating a smart antenna for a self-monitoredre-scan in accordance with the present invention.

FIG. 5 is a flowchart for operating a smart antenna for a periodicre-scan in accordance with the present invention.

FIG. 6 is a flowchart for operating a smart antenna based upon a statusmetric provided by the MAC layer in accordance with the presentivention.

FIG. 7 is a flowchart for operating a smart antenna based upon a powermetric provided by the MAC layer in accordance with the presentivention.

FIG. 8 is a flowchart for operating a smart antenna based upon a timerassociated with the antenna steering algorithm layer in accordance withthe present ivention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in alternative embodiments.

Referring initially to FIGS. 1 and 2, an 802.11 wireless local areanetwork (WLAN) 10 includes an access point 12, and a client station 14operating with a subscriber based smart antenna 16 in accordance withthe present invention. The smart antenna 16, which will also be referredto as a switched beam antenna, generates a plurality of antenna beams inresponse to an antenna steering algorithm 18. The antenna beamsgenerated by the smart antenna 16 include directional beams 20 and anomni-directional beam 22. The illustrated directional beam 20 is aswitched beam for communicating with the access point 12.

The client station 14 includes a beam switching unit 30 connected to thesmart antenna 16, and a transceiver 32 connected to the beam switchingunit. A controller 40 is connected to the transceiver 32 and to the beamswitching unit 30. The controller 40 includes a processor 42 forexecuting the antenna steering algorithm 18. Alternatively, the antennasteering algorithm 18 may operate on an 802.11 PHY/MAC chipset insteadof the illustrated processor 42. The PHY/MAC chipset includes theillustrated PHY layer 43 and the MAC layer 44. Regardless of theprocessor executing the antenna steering algorithm 18, the algorithmutilizes information provided by what is typically called the upper MACor MAC management portion of the MAC software, either via MACabstraction available for access by the external host processor 42 or onthe PHY/MAC chipset.

The use of directional antenna beams 20 improves the throughput of theWLAN 10 and increases the communication range between the access point12 and the client station 14. A directional antenna beam 20 provides ahigh signal-to-noise ratio in most cases, thus allowing the link tooperate at higher data rates. The PHY data rates for 802.11b links are1, 2, 5.5, and 11 Mbps, and the rates for 802.11a are 6, 9, 12, 18, 24,36, 48 and 54 Mbps. The 802.11g devices support the same data rates as802.11a devices as well as the rates supported by 802.11b rates.

The antenna steering algorithm 18, as will be discussed in greaterdetail below, is for 802.11 WLAN client stations, especially those thatsupport 802.11a or 80211g. The algorithm selects antenna beams based oncomputing and tracking of certain quality metrics obtained from the MAClayer management entity (MLME) and the physical layer management entity(PLME). Even though an 802.11 WLAN is discussed with respect to theantenna steering algorithm 18, the algorithm may be adapted to othertypes of local area networks, as readily appreciated by those skilled inthe art.

Although the core logic of the algorithm will be common to animplementation on a PHY/MAC chipset or on the illustrated external hostprocessor 42, there can be differences in performance of the antennasteering algorithm 18 depending on the type of implementation. Forexample, differences could exist between the two types of implementationregarding how fast some of the metrics can be computed, which couldagain result in differences in performance. The antenna steeringalgorithm 18, however, is designed with sufficient parametrization suchthat a single description can be applied to both types ofimplementation.

Referring now to FIG. 3, quality metrics (QM) for selecting antennabeams for the smart antenna 16 are computed. The quality metrics arebased upon signal quality metrics (SQM) and link quality metrics (LQM).For purposes of illustrating the present invention, the smart antenna 16generates 6 directional beams 20 and 1 omni-directional beam 22 for atotal of 7 antenna beams. Each directional beam 20 covers about 60degrees in azimuth.

From the start (Block 300), an initial scan begins at Block 302. Thevariable k represents the current beacon period or time index. Thebeacon periods are provided by the access point 12, as readilyappreciated by those skilled in the art. In the illustrated example,there are 10 beacon periods to accumulate the metrics for each antennabeam to be scanned. Metrics for only 1 antenna beam are determined perbeacon period. Consequently, the flowchart loops through a total of 70beacon periods for the 7 antenna beams, i.e., k ranges from 0 to 69.

After the quality metrics have been determined for each of the 10 beaconperiods for each respective antenna beam, an average quality metric (QM)is determined by a quality metric calculator 50. As will be discussed ingreater detail below, the quality metric calculator 50 includes a signalquality (SQ) module 52 for determining the signal quality metrics and alink quality (LQ) module 54 for determining the link quality metrics.

In Block 304, the antenna beam index n is set to the antenna beam beingevaluated, i.e., n ranges from 1 to 7. The value of n is selected basedupon the remainder of k/N, where N is the number of antenna patterns tobe scanned (i.e., 7) and k is the current beacon period index. Theantenna beam corresponding to the antenna beam index n determined atBlock 304 is held for the beacon period T_(BeaconPeriod) at Block 306.

The beacon period T_(BeaconPeriod) is a periodic or quasi-periodic timeinterval that is typically on the order of 100 msec. In the decisionBlock 308, the current beacon period index k is compared to a numberthat is defined by N*M−1. Since N is the number of antenna patterns toscan (i.e., 7), and M is the number of beacon periods to accumulate themetrics (i.e., 10), k is compared to the number 69 for the illustratedexample.

Each time the current beacon period index k is less than or equal toN*M−1 at Block 308, the method cycles through Blocks 310–318 forcalculating the link quality metric (LQM) and for calculating the signalquality metric (SQM). The beacon period index k is then incremented by 1at Block 320, and the method loops back to Block 304 for the next beaconperiod index n.

In one embodiment, the link quality metric is initially measured at theMAC layer 44 and is based upon the use of several counters 62 therein.The counters 62 are used to provide a MAC Frame Detection Ratio (MFDR),defined as (1−MFER), where MFER is the MAC frame error ratio.

The 802.11 MAC does not have provisions for determining the exact MFDRof all packets that were sent to a client station (downlink) or from aclient station (uplink) solely by looking at the counters 62 that arestandardized in the 802.11 MAC layer 44. Thus, it is not feasible tocompute, for example, the exact downlink (access point 12 to clientstation 14) MFDR. However, there is a way to compute a metric that isrelated to the downlink MPDR and can be a useful metric for measuringdownlink quality.

For example, some of the counters 62 that are defined in the 802.11 MACInformation Base (MIB) may be used to yield an estimate of the linkquality in the downlink, i.e., the link that the client station 14experiences in receiving packets from the access point 12. The MIBcounters 62 of interest for downlink are dot11ReceivedFragmentCount,dot11MulticastFragmentCount, and dot11FCSErrorCount.

The dot11ReceivedFragmentCount, which tracks the number of fragmentsreceived, is any received frame of type data or management of uni-casttype for the purpose of this counter. The steering algorithm 18 tracksthe increment of this counter by Rx_Frag_Cnt(k) for the k-th Beaconperiod.

The dot11MulticastFragmentCount, which tracks the number of multi-castfragments received, is any received frame of type data or management forthe purpose of this counter. The steering algorithm 18 tracks theincrement of this counter by Rx_Mult_Cnt(k) for the k-th Beacon period.

The dot11FCSErrorCount, which tracks the number of frames received, ofany type, which resulted in an FCS error. This counter can also indicatethe link condition of the BSS. The antenna steering algorithm 18 tracksthe increment of this counter by Fcs_Err_Cnt(k) for the k-th Beaconperiod.

The downlink link quality measure (DLQM) is defined as:

$\begin{matrix}{{DLQM} = \frac{\sum\limits_{k}\;{{FCS\_ Err}{\_ Cnt}(k)}}{\sum\limits_{k}\begin{Bmatrix}{{{Rx\_ Frag}{\_ Cnt}(k)} +} \\{{{Rx\_ Mult}{\_ Cnt}(k)} +} \\{{FCS\_ Err}{\_ Cnt}(k)}\end{Bmatrix}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

The DLQM as defined in equation 1 still does not give the accurate frameerror rate in the downlink because i) the denominator counts onlyuni-cast or multi-cast frames of management and data types, while thenumerator counts packet losses due to FCS error of all types of frames,and ii) also the denominator does not distinguish the packet loss due tocollision from pure FCS checksum error.

In fact, the DLQM may over-estimate the downlink frame error rate.However, if such limitations are taken into consideration, for example,by using a higher threshold value to determine acceptable FERperformance than would be used if the DLQM were a more accurateestimator of the FER, the DLQM could still be a useful indicator of thedownlink link quality.

Likewise, a measure of the uplink (client station 14 to access point 12)link quality could be obtained. The MLME counters 62 aredot11ACKFailureCount, and dot11TransmittedFrameCount. Thedot11ACKFailureCount tracks the number of failures in the downlink ACKreception in response to a data packet sent from the client station. Theantenna steering algorithm 18 tracks the increment of this counter byAck_Fail_Cnt(k) for the k-th Beacon period.

The dot11TransmittedFrameCount counts the total number of successfuluplink frame transmissions. A running counter is defined asTx_Form_Cnt(k), where the latter tracks the increments of the MLMEcounter dot11TransmittedFrameCount during any k-th Beacon period.

By using the counter Ack_Fail_Cnt(k) and Tx_Form_Cnt(K) an uplink linkquality metric (ULQM) is obtained. This is an estimate of the uplink MACPacket error rate (MPER) according to:

$\begin{matrix}{{ULQM} = \frac{\sum\limits_{k}{{Ack\_ Fail}{\_ Cnt}(k)}}{\sum\limits_{k}\left\{ {{{Tx\_ Frm}{\_ Cnt}(k)} + {{Ack\_ Fail}{\_ Cnt}(k)}} \right\}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

As in the case of the DLQM (equation 1), the ULQM of equation 2typically over-estimates the actual FCS checksum error rate in theuplink, since the ACK failures in the denominator could be from bothcollision and FCS checksum error at the access point 12. Still, withsuch limitations being considered, the ULQM is useful as a downlinkquality measure.

The search for preferred antenna beams thus uses such estimates of theDLQM and the ULQM. When determining the link quality metric in Block 316based upon the individual downlink and uplinks calculations from Block314, a weighting factor β is used. The weighting factor β is less than1, and is typically selected for emphasizing the downlink calculationsover the uplink calculations, or vice-versa. The weighting factor β isless than 1.

In Block 318 the signal quality metric is determined for the current nand k. Typically, the signal quality metric that is the most readilyavailable from the PHY layer 43 at a driver level is the received signalstrength indicator (RSSI). The RSSI is typically measured at the end ofthe PLCP header for each packet and is provided to the signal qualitymodule 52.

The 802.11 standard defines the RSSI as strictly a relative quantity,i.e., RSSI is not a true measure of the received signal power at anypoint in the receiver. However, depending on the format and frequency ofits availability, the RSSI can still be a useful metric on which to basethe antenna steering algorithm 18. The RSSI will hold not only forreception but also for transmission, although to a lesser degree, sincein the 802.11 WLAN the wireless physical channel is a shared media forboth downlink and uplink. Of course, a signal-to-noise (SNR) ratio mayalso be used.

When the beacon period index k exceeds N*M−1 in Block 308, the methodcontinues to Block 322 for determining a weighting factor α for thesignal quality metric. The weighting factor α is less than 1, and istypically selected for emphasizing the link quality metric over thesignal quality metric. The quality metric QM for each antenna beam indexn and for each beacon period index k is calculated in Block 324. Sincethere are 10 quality metric calculations for each antenna beam, anaverage quality metric value is obtained in Block 326. Based upon theaverage quality metric value for each antenna beam, the antenna beam nwith the highest value is selected in Block 328.

Candidate or alternate antenna beams are further selected in Block 330based upon the quality metric values determined in Block 328. In otherwords, the antenna beam n_(c1) with the second highest quality metricvalue is selected, as well as the antenna beam n_(c2) with the thirdhighest quality metric value. As a default, one of the alternate antennabeams is an omni-directional beam 22 if the preferred antenna beam is adirectional beam 20. Once the preferred and alternate antenna beams havebeen selected, the system goes to a sustained use state or period inBlock 332. In the sustained use state or period, the selected antennabeam is used at the client station 14 for both downlink and uplink, forall frames, in the next P_(SU) presumed beacon periods, where60<P_(SU)<6000 and has a default value of 600. The method ends at Block334.

The link quality metric is thus computed to augment and improve on theantenna steering decision in addition to the signal quality metric. Thelink quality metrics are based on information available from fiveexisting counters operated in the 802.11 Media Access Control (MAC)processes. As noted in Block 304, two separate estimates of the frameerror rates (FER) are obtained, one is the downlink quality metric(DLQM) and the other is the uplink quality metric (ULQM). The 802.11WLAN media access control (MAC) layer management entity (MLME) providesthe frame counters to estimate the DLQM and ULQM.

As an alternative to using the FER-based link quality metric, LENGTH(i.e., throughput) and RATE information provided by the MAC layer 44 maybe used. The LENGTH and RATE information can be obtained from the 802.11MAC layer 44 for each of the transmitted or received MAC frames. A RATEmodule 64 and a LENGTH module 64 are used to provide estimates of theMAC-layer transfer rates in both downlink (receive side) and uplink(transmit side). Such estimated transfer rates are computed from theLENGTH and RATE information per transmitted or received MAC frames overa period of time.

The antenna steering algorithm 18 has at least driver-level read accessto RATE_(TX) (m,k) within the MAC layer 44 that reports the RATE, inunit of Mbps, of the m-th received frame in the k-th presumed beaconperiod at the end of each of the k-th presumed beacon period within areasonable latency. The RATE may also be computed in the uplink. Theantenna steering algorithm 18 also has at least driver-level read accessto SIZE_(RX) (m,k) within the MAC layer 44 that reports the SIZE inbytes of the m-th received frame in the k-th presumed beacon period atthe end of each of the k-th presumed beacon period within a reasonablelatency.

Other aspects of the antenna steering algorithm 18 are directed tomethods for performing a self-monitored re-scan and a periodic rescan.The self-monitored re-scan involves monitoring the currently selectedantenna beam, whereas the periodic re-scan involves monitoring alternateantenna beams.

The self-monitored re-scan is performed by the antenna steeringalgorithm 18 during a sustained use period after having selected apreferred antenna beam. During a sustained use period, a re-scan of theother antenna beams is not performed. The antenna steering algorithm 18periodically monitors a quality metric of the ongoing radio linkprovided by the preferred antenna beam. The quality metric is based upona signal quality metric and a link quality metric. If the quality metricdeteriorates below certain thresholds during the sustained use period,the steering algorithm 18 either swaps the preferred antenna beam withan alternate antenna beam or initiates a re-scan of the availableantenna beams for selecting a new preferred antenna beam.

As stated above, during any sustained use period, if a self-monitoredre-scan trigger event happens, the antenna steering 18 performs aself-monitored re-scan. During the sustained use period, aselected-pattern quality metric is computed from metric data from theM_(SP) most recent presumed beacon periods and is evaluated at the endof every M_(SP)/2 presumed beacon periods. M_(SP) is an even integerlarger than 0 and smaller than 12 and has a default value of 6, forexample.

The self-monitored re-scan trigger event is defined as an event wherethe current selected pattern quality metric takes on a value that islower by some threshold values compared to the average value of the samemetric in the last M_(AVG) most recent previous evaluation periods.Depending on the amount that the selected-pattern quality metric dropscomparatively with the average value, either the current selectedpattern would be swapped with a candidate pattern identified earlier, ora re-scan of all N patterns will occur. Also, when a self-monitoredre-scan occurs, the timer for the sequencing of periodic re-scan and thesustained use period is reset, and a new sustained use period of lengthP_(SU) presumed beacon periods starts.

Referring now to FIG. 4, a flowchart for steering a smart antenna 16using the self-monitored re-scan will be discussed. From the start(Block 400), a preferred antenna beam and an alternate antenna beam areselected at Block 402. Data is exchanged with the access point 12 atBlock 404 using the preferred antenna beam during a sustained use periodduring which a re-scan of the other antenna beams is not performed.

During the sustained use period a quality metric of the exchanged datais periodically calculated at Block 406 for the preferred antenna beam.The calculating comprising determining at least one link quality metric(LQM) of the exchanged data for the preferred antenna beam at Block 408.A signal quality metric (SQM) of the exchanged data for the preferredantenna beam is determined at Block 410. The at least one LQM and theSQM are combined at Block 412 for calculating the quality metric. Thequality metric for the preferred antenna beam is compared to a swapthreshold range at Block 414.

The preferred antenna beam is swapped with the alternate antenna beam atBlock 416 if the quality metric is within the swap threshold range tocontinue exchanging data with the access point 12 within the sustaineduse period. If the quality metric is not within the swap thresholdrange, then the quality metric is compared to a re-scan threshold atBlock 418 for initiating a re-scan of the plurality of antenna beams forselecting a new preferred antenna beam. The method ends at Block 420.

The periodic re-scan is performed by the antenna steering algorithm 18at an end of a sustained use period and before a next sustained useperiod. During a sustained use period, a re-scan of the other antennabeams is not performed. The periodic re-scan is performed on alternateantenna beams that were selected when the preferred antenna beam wasselected.

The antenna steering algorithm 18 monitors a quality metric of thealternate antenna beams as well as a quality metric for the preferredantenna beam. If the quality metric of the preferred antenna beam isless than the quality metrics of anyone of the alternate antenna beams,then the alternate antenna beam corresponding to the quality metrichaving a higher value is selected for the next sustained use period.

As stated above, if a self-monitored re-scan does not take place duringthe preceding sustained-use period, a periodic re-scan takes place. Aperiodic re-scan decision metric is computed on the alternate antennabeams for (N_(C)+1)*M presumed beacon periods, where N_(C) is the numberof candidate or alternate antenna beams. If the present selected antennabeam is omni-directional, then the remaining alternate antenna beamswill be directional beams. If the switch beam antenna 16 has 7 antennabeams and the currently selected antenna beam is a directional antennabeam, then one of the alternate antenna beams will be theomni-directional beam 22 and the other alternate antenna beam will be adirectional antenna beam 20.

During the periodic re-scan period, the antenna beams are scanned on allframes received or transmitted on all the alternate antenna beams.Subsequently, a decision on whether to replace or maintain the existingselected antenna beam will be made. A new sustained-use period of lengthP_(SU) presumed beacon periods follows, after which another periodicre-scan takes place. This regular, periodic sequence of periodic re-scanand sustained use is continued, except when a self-monitored re-scantrigger event or a RSSI-drop induced re-scan takes place during thepreceding sustained-use period.

Referring now to FIG. 5, a flowchart for steering a smart antenna 16using a periodic re-scan wil now be discussed. From the start (Block500), a preferred antenna beam and at least one alternate antenna beamare selected at Block 502. Data is exchanged with the access point 12 atBlock 504 using the preferred antenna beam during a sustained use periodduring which a re-scan of the other antenna beams is not performed.

At an end of the sustained use period and before a next sustained useperiod, a quality metric of exchanged data for the preferred antennabeam and for each alternate antenna beam is calculated at Block 506. Thecalculating comprises determining at least one link quality metric (LQM)of the exchanged data for the preferred antenna beam at Block 508. Asignal quality metric (SQM) of the exchanged data for the preferredantenna beam is determined at Block 510. The at least one LQM and theSQM are combined at Block 512 for calculating the quality metric for thepreferred antenna beam. The determining and combining are repeated atBlock 514 for calculating a quality metric for each alternate antennabeam.

The quality metric for the preferred antenna beam is compared to thequality metrics for the alternate antenna beams at Block 516. If thequality metric for the preferred antenna beam is less than at least oneof the quality metrics for the alternate antenna beams, then thealternate antenna beam corresponding to the at least one quality metrichaving a higher value is selected at Block 518 to continue exchangingdata with the access point 12 within the next sustained use period. Themethod ends at Block 520.

Another aspect of the present invention is to operate the antennasteering algorithm 18 in response to three functions the MAC layerperforms. The functions are notification of a change in the MAC_STATUS,MAC_PowerMode and Beacon Period Synchronization Information. Thefollowing modules within the MAC layer 44 are associated with thesefunctions: Status 72, Power 74 and Synchronization 76.

The MAC_STATUS function 72 and the MAC_PowerMode 74 function notify theantenna steering algorithm 18 of changes in the MAC states within theMAC layer 44. The notification ensures that the MAC states areappropriate so that the antenna steering algorithm 18 will operateaccordingly. The Beacon Period Synchronization Information 76 used bythe MAC layer 44 allows the antenna steering algorithm 18 to maintaintight synchronization with the actual beacon periods.

The MAC layer 44 within the client station 14 communicates with theantenna steering algorithm 18 to decide antenna beam selection. Duringantenna beam selection periods, the main functions of the MAC layer 44involve several MAC state machines, such as AuthreqService_Sta,AuthRspService_Sta, AsocService_Sta and Synchronization_Sta running insuccession in the MAC layer 44 during the start-up time of the clientstation 14.

The antenna steering algorithm 18 itself just needs know the status ofthe MAC state machine on ready service or not, instead of the moredetailed MAC functions and procedures. Thus, an abstracted status metricMAC_STATUS 72 is defined at the MAC layer 44 to compute the necessaryabstracted information. When the value of the MAC_STATUS changes, theMAC layer 44 notifies the antenna steering algorithm 18 to verify thestate of the MAC_STATUS 72. The MAC_STATUS metric is computed asprovided in equation 3. The antenna steering algorithm 18 then respondsto the changes in the MAC_STATUS state.

$\begin{matrix}{{MAC\_ STATUS} \equiv {\quad\left\lbrack \begin{matrix}{{0,{{if}\mspace{14mu} S_{SYNCH\_}{STATUS}\mspace{14mu}{AND}}}\mspace{14mu}} \\{{S_{ASSOCIATION\_}{STATUS}\mspace{14mu}{AND}\mspace{14mu} S_{AUTH\_}{STATUS}} = 0} \\{{1,{{if}\mspace{14mu} S_{SYNCH\_}{STATUS}\mspace{14mu}{AND}}}\mspace{14mu}} \\{{S_{ASSOCIATION\_}{STATUS}\mspace{14mu}{AND}\mspace{14mu} S_{AUTH\_}{STATUS}} = 1}\end{matrix} \right.}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

Three different statuses of the MAC_STATUS 72 are monitored tosynchronize the antenna beam selection functions and the MAC statemachine. The three different statuses are S_(SCANNING),S_(AUTHENTICATION) and S_((RE) ASSOCIATION).

The S_(SCANNINGN) _(—) STATUS state indicates that the client station 14has either successfully synchronized or unsynchronized to an accesspoint 12. This status may also be termed as a BSS state. The status willbe 1 if the client station 14 has passed the access pointsynchronization. Otherwise, the status will be 0.

From the 802.11 standards, all data frames will be undeliverable on bothuplink and downlink if the MAC state machine runs out of the BSS state.In this case, the MAC layer 44 only receives the beacon frame, butrejects any application data frames. Therefore, the state of BSS is usedas a condition to start the antenna beam selection.

The S_(ASSOCIATION) _(—) STATUS state indicates that the client station14 has either successfully associated or un-associated to an accesspoint 12. This status may also be termed as an assoc state. The statuswill be 1 if the client station 14 passes the access point association.Otherwise, the status will be 0.

The S_(AUTH) _(—) STATUS state indicates that the client station 14 hassuccessfully passed either authentication or de-authentication. Thisstatus is termed as an auth_open state or an auth_key state in the802.11 standards. The status will be 1 if the client station 14 haspassed authentication. Otherwise, the status will be 0.

From the 802.11 standards, the authentication service is used by allclient stations 14 to establish their identity to the access point 12with which they will communicate. Two types of authentication serviceare open system and shared key. The open system authentication violatesimplicit assumptions made by higher network layers. The MAC layer 44just verifies the MAC address. The shared key authentication requiresimplementation of the wired equivalent privacy (WEP) option, and theidentity is demonstrated by knowledge of a shared, secret, WEPencryption key. Regardless of the type of authentication service to beused, a status result from authentication processing will be used ascondition to start the antenna beam selection.

The authentication process may be time-consuming depending on theauthentication protocol in use. The authentication service can beinvoked independently of the association service. A client station 14already associated with an access point (with which it previouslyauthenticated) typically performs preauthentication. However, the 802.11standards do not require client stations 14 to preauthenticate withaccess points 12, but authentication is required before an associationcan be established.

When all three management procedures, i.e., scan, authentication, andassociation are achieved, the MAC_STATUS 72 is set to 1. The MAC layer44 then notifies the antenna steering algorithm 18 of the change. Theantenna steering algorithm 18 then sets its SCAN_STATE to 1, i.e.,during initial scan, and starts an initial scan procedure as describedabove. Also, subsequent operations of the antenna steering algorithm 18take place, such as a sustained use period or different types ofre-scans.

If any of the three status metrics become 0, the value of MAC_STATUS 72changes to 0. This change is again notified from the MAC layer 44 to theantenna steering algorithm 18. The antenna steering algorithm 18subsequently resets a currently selected antenna beam to a defaultantenna beam, such as the omni-directional antenna beam 22. The antennasteering algorithm 18 also resets its timers to right before the startof an initial scan, and resets its SCAN_STATE to 0, i.e., before initialscan or start-up.

Referring to the flowchart illustrated in FIG. 6, selecting antennabeams by the antenna steering algorithm 18 in response to notificationof a change of the MAC_STATUS 72 will now be discussed. From the start(Block 600), the client station 14 is placed in a powered on state atBlock 602. A status metric 72 is computed at Block 604. The statusmetric 72 indicates a status of the following events: synchronization ofthe client station 14 with the access point 12, association of theclient station with the access point, and authentication of the clientstation by the access point. The status metric 72 has a first value whenthe events are met and a second value when anyone of the events is notmet.

The plurality of antenna beams are scanned at Block 614 for selecting apreferred antenna beam for exchanging data with the access point 12 whenthe status metric 72 has the first value. The status metric 72 ismonitored at Block 616. The preferred antenna beam is changed to adefault antenna beam at Block 618 when the status metric 72 changes fromthe first value to the second value. The method ends at Block 620.

The MAC layer also computes and maintains a power metric S_(POWER) _(—)STATUS 74. The value of S_(POWER) _(—) STATUS 74 is used in notificationof a change in the power-saving mode status. The power metric 74 isupdated by the MAC layer 44 via reading the MAC transmissioncoordination state machine (Tx-Coordination). Upon a change of the valueof this metric, the MAC layer 44 notifies the antenna steering algorithm18.

A state of the power metric S_(POWER) _(—) STATUS 74 indicates that theclient station 14 has either wakened up or has been moved into a powersave mode. This function is termed as a TxC_Idle state or an Asleepstate in the 802.11 standard. The status will be 1 if the client stationwakes up. Otherwise, the status will be 0. The power metric S_(POWER)_(—) STATUS 74 is computed as provided in equation 4.

$\begin{matrix}{{S_{POWER\_}{STATUS}} \equiv \left\{ \begin{matrix}{0,} & {{{if}\mspace{14mu}{CS}\mspace{14mu}{is}\mspace{14mu}{in}\mspace{14mu}{Power}\mspace{14mu}{Save}\mspace{14mu}{Mode}},} \\{1,} & {{if}\mspace{14mu}{CS}\mspace{14mu}{is}\mspace{14mu}{in}\mspace{14mu}{Normal}\mspace{14mu}{Power}\mspace{14mu}{Mode}}\end{matrix} \right.} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

The Tx-Coordination state machine in the MAC layer 44 uses the state atall stations. The MAC layer 44 turns off both the transmitter and thereceiver during the power saving mode, and the MAC layer ramps up thereceiver power prior to the TBTT. The client station 14 retains itscurrent power management mode until it informs the access point 12 via asuccessful frame exchange that it is going to wake up. The status of thepower metric S_(POWER) _(—) STATUS 74 is used to instruct the antennasteering algorithm 18 to either resume a normal antenna steeringoperation or to turn off the operations.

In particular, if the power metric S_(POWER) _(—) STATUS 74 changes from1 to 0, i.e., the client station 14 goes into a power saving mode, thenthe antenna steering algorithm 18 saves the indices for the currentselected antenna beam and any alternate antenna beam. The antennasteering algorithm 18 also resets its timer to the end of a sustaineduse period or to the start of a new periodic re-scan, and then sets itsSCAN_STATE, and notifies the MAC layer 44 on the completion of theseprocedures. If the power metric S_(POWER) _(—) STATUS 74 changes from 0to 1, i.e., the antenna steering algorithm 18 immediately performs theperiodic re-scan using the last saved selected antenna beam and thealternate antenna beam.

Referring to the flowchart illustrated in FIG. 7, selecting antennabeams by the antenna steering algorithm in response to notification of achange of the S_(POWER) _(—) STATUS 74 will now be discussed. From thestart (Block 700), the client station 14 is placed in a powered on stateat Block 702. A power metric 74 is computed at Block 704. The powermetric 74 has a first value indicating that the client station 14 is inthe powered on state, and a second value indicating that the clientstation is in a power savings state.

The plurality of antenna beams are scanned at Block 706 for selecting apreferred antenna beam and at least one alternate antenna beam forexchanging data with the access point 12 when the power metric 74 hasthe first value. The power metric 74 is monitored at Block 708 for achange from the first value to the second value. At Block 710,selections of the antenna beams are stored for the preferred antennabeam and for the at least one alternate antenna beam when the powermetric 74 changes to the second value indicating that the client station14 is in the power savings state. The method ends at Block 712.

A Beacon Period Synchronization Information Timer metric T_(bcn) 14 isalso defined, computed and maintained at the antenna steering algorithm18 to better synchronize its timing with the actual timer of the MAClayer 44, and resultantly, to have better synchronization of thepresumed beacon period of the antenna steering algorithm with the actualbeacon periods.

The Beacon Period Information Time Metric T_(bcn) 76 is a counter thattracks the presumed beacon interval of the antenna steering algorithm18. When this counter reaches a certain pre-specified number, theantenna algorithm 18 queries the MAC layer 44 and fetches the value ofthe MAC TSF. The antenna steering algorithm 18 can then use the fetchedMAC timer value to update its own timer. This timer is used in theantenna steering algorithm 18 to align the searching time with thebeacon period.

The antenna steering algorithm 18 updates synchronization to the actualbeacon periods periodically, instead of during each beacon period.In-between the updating periods, the antenna steering algorithm 18maintains a timer for the presumed beacon period interval, and runs thetimer for each beam searching period. The antenna steering algorithm 18updates the boundary of the presumed beacon period when it receives anupdate input from the MAC layer 44. On the k-th presumed beacon period,the timer value for the current presumed beacon period is computed asprovided in equation 5.

$\begin{matrix}{{T_{BCN}(k)} \equiv \begin{Bmatrix}{0,{{{if}\mspace{14mu} k} = {{{rem}\left( {k,{M \cdot {TU}}} \right)} \neq 0}}} \\{{{BeaconPeriod}({integer})},{{{if}\mspace{14mu} k} = {{{rem}\left( {k,{M \cdot {TU}}} \right)} = 0}}}\end{Bmatrix}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

The M*TU value is a number chosen to signify the length of time that thetimer of the antenna steering algorithm 18 can operate without beingupdated by the MAC timer. The rem(x, y) is a remainder of integer x whendivided by integer y. The TU is a time unit whose length is 1024 μs.Since TBTT is usually 100 TU (100 msec), M should be at least a multipleof 100, such as 500 or 1000, for example.

Referring to the flowchart illustrated in FIG. 8, selecting antennabeams by the antenna steering algorithm in response to a timermaintained by the algorithm 18 will now be discussed. From the start(Block 800), the client station 14 receives beacon frames from theaccess point for setting a first beacon timer tracking beacon periods ofthe received beacon frames at Block 802. The first beacon timer isoperated separate from the antenna steering algorithm 18. The firstbeacon timer is periodically synchronized with a second beacon timer atBlock 804 that is operated at the antenna steering algorithm 18 for alsotracking the beacon periods of the received beacon frames. The secondbeacon timer is run during each antenna beam searching period at Block806. The method ends at Block 808.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings. Inaddition, other features relating to smart antennas are disclosed incopending patent applications filed concurrently herewith and assignedto the assignee of the present invention and are entitled METHOD FORSTEERING SMART ANTENNA BEAMS FOR A WLAN USING SIGNAL AND LINK QUALITYMETRICS, Ser. No. 11/080,038, METHOD FOR STEERING SMART ANTENNA BEAMSFOR A WLAN USING MAC LAYER FUNCTIONS, Ser. No. 11/080,039, METHOD FORSTEERING A SMART ANTENNA FOR A WLAN USING A PERIODIC RE-SCAN, Ser. No.11/080,317, the entire disclosures of which are incorporated herein intheir entirety by reference. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed,and that modifications and embodiments are intended to be includedwithin the scope of the appended claims.

1. A method for operating a client station in a wireless local areanetwork (WLAN) communication system comprising an access point, theclient station comprising an antenna steering algorithm and a smartantenna responsive to the antenna steering algorithm for selecting oneof a plurality of antenna beams, the method comprising: selecting apreferred antenna beam and an alternate antenna beam; exchanging datawith the access point using the preferred antenna beam during asustained use period during which a re-scan of the other antenna beamsis not performed; periodically calculating during the sustained useperiod a quality metric of the exchanged data for the preferred antennabeam, the calculating comprising determining at least one link qualitymetric (LQM) of the exchanged data for the preferred antenna beam,determining a signal quality metric (SQM) of the exchanged data for thepreferred antenna beam, and combining the at least one LQM and the SQMfor calculating the quality metric; comparing the quality metric for thepreferred antenna beam to a swap threshold range; and swapping thepreferred antenna beam with the alternate antenna beam if the qualitymetric is within the swap threshold range to continue exchanging datawith the access point within the sustained use period.
 2. A methodaccording to claim 1 wherein if the quality metric is not within theswap threshold range, then further comprising comparing the qualitymetric to a re-scan threshold for initiating a re-scan of the pluralityof antenna beams for selecting a new preferred antenna beam.
 3. A methodaccording to claim 1 wherein the alternate antenna beam comprises anomni-directional beam.
 4. A method according to claim 1 wherein the WLANcomprises an 802.11 WLAN.
 5. A method according to claim 1 wherein theat least one LQM is based upon at least one estimate of a frame errorrate (FER) of the exchanged data.
 6. A method according to claim 1wherein the at least one LQM comprises a downlink LQM and an uplink LQM.7. A method according to claim 6 wherein a weighting factor is used whencombining the downlink LQM and the uplink LQM.
 8. A method according toclaim 6 wherein the WLAN comprises an 802.11 WLAN and the client stationcomprises a media access control (MAC) layer including a plurality offrame counters for estimating frame error rates of the exchanged data;and wherein a first set of counters is used for determining the downlinkLQM and a second set of counters is used for determining the uplink LQM.9. A method according to claim 1 wherein the at least one LQM is basedupon a transfer rate of the exchanged data for the corresponding antennabeam.
 10. A method according to claim 1 wherein the at least one LQM isbased upon at least one of a throughput and a data rate of the exchangeddata for the corresponding antenna beam.
 11. A method according to claim1 wherein the SQM is based upon a received signal strength indicator(RSSI) of the exchanged data.
 12. A method according to claim 1 whereina weighting factor is used when combining the LQM and the SQM.
 13. Amethod according to claim 1 wherein the plurality of antenna beamscomprise a plurality of directional beams and an omni-directional beam.14. A client station for operating in a wireless local area network(WLAN) communication system comprising an access point, the clientstation comprising: a switched beam antenna for generating a pluralityof antenna beams; a beam switching unit coupled to said switched beamantenna for selecting a preferred antenna beam and an alternate antennabeam; a transceiver coupled to said beam switching unit for exchangingdata with the access point via a preferred antenna beam during asustained use period during which a re-scan of the other antenna beamsis not performed; an antenna steering algorithm module for running anantenna steering algorithm for periodically calculating during thesustained use period a quality metric of the exchanged data for thepreferred antenna beam, the calculating comprising determining at leastone link quality metric (LQM) of the exchanged data for the preferredantenna beam, determining a signal quality metric (SQM) of the exchangeddata for the preferred antenna beam, and combining the at least one LQMand the SQM for calculating the quality metric; and said antennasteering algorithm module comparing the quality metric for the preferredantenna beam to a swap threshold range, and swapping the preferredantenna beam with the alternate antenna beam if the quality metric iswithin the swap threshold range to continue exchanging data with theaccess point within the sustained use period.
 15. A client stationaccording to claim 14 wherein if the quality metric is not within theswap threshold range, then said antenna steering algorithm modulefurther compares the quality metric to a re-scan threshold forinitiating a re-scan of the plurality of antenna beams for selecting anew preferred antenna beam.
 16. A client station according to claim 14wherein the alternate antenna beam comprises an omni-directional beam.17. A client station according to claim 14 wherein the WLAN comprises an802.11 WLAN.
 18. A client station according to claim 14 wherein the atleast one LQM is based upon at least one estimate of a frame error rate(FER) of the exchanged data.
 19. A client station according to claim 14wherein the at least one LQM comprises a downlink LQM and an uplink LQM.20. A client station according to claim 19 wherein a weighting factor isused when combining the downlink LQM and the uplink LQM.
 21. A clientstation according to claim 19 wherein the client station comprises amedia access control (MAC) layer including a plurality of frame countersfor estimating frame error rates of the exchanged data; and wherein afirst set of counters is used for determining the downlink LQM and asecond set of counters is used for determining the uplink LQM.
 22. Aclient station according to claim 14 wherein the at least one LQM isbased upon a transfer rate of the exchanged data for the correspondingantenna beam.
 23. A client station according to claim 14 wherein the atleast one LQM is based upon at least one of a throughput and a data rateof the exchanged data for the corresponding antenna beam.
 24. A clientstation according to claim 14 wherein the SQM is based upon a receivedsignal strength indicator (RSSI) of the exchanged data.
 25. A clientstation according to claim 14 wherein a weighting factor is used whencombining the LQM and the SQM.
 26. A client station according to claim14 wherein the plurality of antenna beams comprise a plurality ofdirectional beams and an omni-directional beam.